The present invention relates generally to hot melt adhesive dispensing systems, and more particularly to a new and improved snuffback valve for dispensing hot melt adhesive whereby quick shutoff of the adhesive supply is able to be achieved at the end of a dispensing operation and during closure of the valve such that undesirable stringing of the adhesive does not occur.
In connection with the dispensing of highly-viscous materials, such as, for example, hot melt adhesives, it is imperative that the dispensing system comprise what is known in the art as a snuffback valve by means of which shut off of the dispensed adhesive is readily achieved upon closure of the valve whereby stringing of the adhesive does not occur. One type of known PRIOR ART snuffback valve is disclosed in FIG. 1 wherein the snuffback valve is generally indicated by the reference character 10. The snuffback valve 10 is adapted to be fixedly mounted upon a valve stem, note shown, which, in turn, is adapted to be fixedly connected, for example, to a piston drive, also not shown, by means of which the snuffback valve is moved linearly with respect to its valve seat member, not shown, so as to attain its relative opened and closed positions with respect to the valve seat.
More particularly, the PRIOR ART snuffback valve 10 comprises a substantially cylindrical structure having an axis 11, and is seen to comprise a first bore 12 and a second counterbored portion 13 with a shoulder 14 defined therebetween. The valve stem, not shown, extends through the counterbored portion 13 and the valve end of the stem is seated upon the shoulder portion 14. A bolt type fastener, not shown, extends through bore 12 and threadedly engages the valve end of the valve stem, not shown, so as to fixedly mount the valve stem within the valve 10. The valve 10 further comprises a cylindrical throat portion 15 which has an external peripheral cylindrical surface portion 16 wherein the external surface portion 16 is adapted to be disposed internally within the valve seat member, not shown, when the snuffback valve 10 is moved to its closed position so as to cooperate with the valve seat member in terminating flow of the adhesive. An annular tolerance, gap, or space, of, for example, 0.010 inches, is defined between the external cylindrical surface portion 16 of the snuffback valve 10 and the internal peripheral cylindrical surface portion of the valve seat member, not shown, so as to readily permit linear movement of the snuffback valve 10 with respect to its valve seat member, not shown, during opening and closing cycles of the valve, as well as to define or determine the snuffback action of the valve 10. Downstream of the throat portion 15, the snuffback valve 10 comprises an external frusto-conical surface portion 18 which is adapted to be mated with a complimentarily shaped frusto-conical surface portion of the valve seat member, not shown, when the snuffback valve 10 is disposed at its closed position and is therefore seated upon the valve seat member.
Still further, the snuffback valve 10 comprises a second cylindrical portion 20 having an external peripheral cylindrical surface portion 22 which is disposed downstream of the frusto-conical portion 18 as considered in the dispensing direction of the hot melt adhesive. The external diametrical extent of the second peripheral cylindrical surface portion 20 is substantially greater than that of the throat portion 15 such that the external peripheral cylindrical surface portion 22 of the second cylindrical portion 20 cooperates with an internal peripheral cylindrical surface portion of a valve seat adapter or dispensing block, not shown, so as to define a discharge path for the adhesive to be dispensed. It is lastly noted that the front or down-stream face 24 of the snuffback valve 10 is disposed perpendicular to the axis 11 of the snuffback valve 10, and that the diametrical extent of the front or downstream face 24 of the snuffback valve 10 is such as to extend radially outwardly with respect to the axis 11 and has an external diametrical extent which is substantially equivalent to that of the second cylindrical portion 20.
While the aforenoted PRIOR ART type of snuffback, valve 10 has performed substantially satisfactorily, the structure of the aforenoted PRIOR ART snuffback valve 10 has in fact presented several operational problems or drawbacks. For example, it is firstly noted that the throat portion 15 of the PRIOR ART snuffback valve 10 has a substantially elongated axial length dimension. In addition, the external diametrical extent of the throat portion 15 is such that, as has been noted hereinbefore, a relatively large clearance of, for example, 0.010 inches, is defined between the external peripheral cylindrical surface portion 16 of the snuffback valve 10 and the internal peripheral cylindrical surface portion of the valve seat member, not shown. Consequently, while the snuffback valve 10 is of course able to achieve its closed position with respect to the valve seat, not shown, when the snuffback valve 10 is appropriately energized by its associated drive mechanism or system, not shown, such that dispensing of the adhesive by the valve 10 is terminated, the stroke of the snuffback valve 10 is considerably long, thereby rendering the speed of the valve relatively slow. This operative feature, which also results in more extensive wear of the seal members and therefore a shortening of the seal life, along with the relatively large tolerance or clearance space defined between the external peripheral cylindrical surface portion 16 of the throat portion and the internal peripheral cylindrical surface portion of the valve seat member, delays the onset or achievement of the snuffback operation of the valve thereby permitting stringing of the adhesive to nevertheless occur.
In addition, due to the provision or structure of the front or downstream surface or face 24 of the snuffback valve 10 as being disposed perpendicular to the axis 11 of the valve 10, and in addition, in view of the fact that such surface or face 24 extends radially outwardly so as to have substantially the same external diametrical extent as that of the second cylindrical portion 20, the snuffback valve 10 causes a phenomenon, known as bursting, to occur when a new adhesive dispensing cycle is commenced. Bursting is the sudden expulsion, discharge, or dispensing, upon the commencement of a new adhesive dispensing operation or cycle, of a predetermined amount or glob of adhesive which residually remains disposed between the downstream end or front face of the snuffback valve and the exit orifice, not shown, of the adhesive dispensing mechanism when a previous dispensing operation or cycle is terminated. The front face or surface 24 of the snuffback valve 10 therefore acts, in effect, as a type of plow forcing the glob or residual adhesive to be suddenly dispensed as a result of the linear movement of the valve 10 attendant the opening of the valve 10 and the unseating of the throat portion 15 of the valve 10 with respect to the valve seat member upon commencement of a subsequent dispensing cycle or operation.
A need therefore exists in the art for a new and improved snuffback valve wherein the structure of the valve is such that when dispensing of, for example, highly viscous materials, such as, for example, hot-melt adhesives, is to be achieved, the various operational drawbacks and disadvantages characteristic of such dispensing systems, such as, for example, stringing and bursting, do not occur, and in addition, the valve is characterized by means of a relatively short movement stroke with a relatively quick operational response time.
Accordingly, it is an object of the present invention to provide a new and improved snuffback valve for use in connection with the dispensing of highly viscous materials, such as, for example, hot melt adhesives.
Another object of the present invention is to provide a new and improved snuffback valve for use in connection with the dispensing of highly viscous materials, such as, for example, hot melt adhesives, which can effectively overcome the various operational drawbacks of conventional snuffback valves.
An additional object of the present invention is to provide a new and improved and improved snuffback valve for use in connection with the dispensing of highly viscous materials, such as, for example, hot melt adhesives, which can effectively prevent the occurrence of bursting and stringing upon the commencement and termination of dispensing operations.
A further object of the present invention is to provide a new and improved snuffback valve for use in connection with the dispensing of highly viscous materials, such as, for example, hot melt adhesives, which also exhibits a relatively short operational stroke and attendant response time during both the opening and closure movements of the valve member with respect to its valve seat, and in, addition, increased service life of the seal members.
The foregoing and other objectives are achieved in accordance with the principles and teachings of the present invention through the provision of a new and improved snuffback valve which comprises a poppet type valve which has a leading or downstream end portion having a substantially frusto-conical configuration whereby such leading or down-stream end portion is effectively aerodynamic with respect to the highly viscous material such that the occurrence of bursting, as characteristic of PRIOR ART snuffback valves, is effectively eliminated. In addition, the snuffback valve of the present invention comprises a trailing or upstream end portion which also has a substantially frusto-conical configuration, and the trailing or upstream frusto-conically configured end portion also has associated therewith an annular shoulder portion disposed substantially perpendicular to the longitudinal axis of the poppet valve. The shoulder portion thus forms a sharp, right-angled edge region with respect to the large-diameter main cylindrical portion of the poppet valve, and in addition, the shoulder portion also forms in effect a rearwardly facing plow portion. These two structural features of the shoulder portion serve to disturb the flow parameters or characteristics of the viscous material, and in addition, serve to establish and maintain partial vacuum conditions within the poppet valve chamber during the valve closing cycle in such a manner that, together with the fact that a substantially small tolerance or gap is defined between the throat portion of the poppet valve and the valve seat member, the phenomenon of stringing is likewise effectively eliminated.